1. Field of the Invention
The present invention relates to a unit pixel of an image sensor, and more particularly, to a 4 transistors and 4 shared (hereinafter “4T-4S”) step and repeat unit pixel obtained by combining four unit pixels into a single one and an image sensor having the same.
2. Description of the Related Art
An image sensor is obtained by arranging a plurality of unit pixels in a 2-dimensional array structure. A process of repeatedly arranging unit pixels in both of horizontal and vertical directions in a design stage of the image sensor is called a step & repeat process. In this process, a horizontal unit line is formed by repeatedly arranging a single type of unit pixels in a horizontal direction, and vertical unit lines are extended from each unit pixel of the horizontal unit line by repeatedly arranging the unit pixels in a vertical direction, thereby implementing a single image sensor.
A 4T unit pixel, a 4T-2S unit pixel, and a 4T-4S unit pixel, which will be referred herein can be defined as follows.
The 4T unit pixel includes a single photodiode and an image signal conversion circuit having four transistors (i.e., 4T) for processing image signals detected from the photodiode.
The 4T-2S unit pixel is an image sensor unit cell obtained by combining two 4T unit pixels into a single one by sharing the transistors (i.e., 2S) used for the image signal conversion circuit.
The 4T-4S unit pixel is an image sensor unit cell obtained by combining four 4T unit pixels into a single one by sharing the transistors (i.e., 4S) used for the image signal conversion circuit.
FIG. 1 illustrates a process of creating a conventional 4T-2S step & repeat unit pixel.
Referring to FIG. 1, the 4T-2S step & repeat unit pixel 100 shown in the right-hand side is obtained by combining two 4T unit pixels 10 and 20 shown in the left-hand side. A first unit pixel 10 includes a single photodiode PD1 and an image signal conversion circuit having four transistors M11, M12, M13 and M14 for converting the image signals generated from the photodiode PD1 into electrical signals. A second unit pixel 20 has a similar structure to the first unit pixel 10, but their components are denoted by different symbols in order to distinguish them from one another.
In the 4T-2S step & repeat unit pixel 100, a reset transistor M12 or M22, a conversion transistor M13 or M23, and a selection transistor M14 or M24 are shared between two 4T unit pixels 10 and 20, each of which has four transistors. Therefore, the number of transistors required for the reset, conversion, and selection transistors can be reduced to three if two 4T unit pixels share them, while six transistors are required if two 4T unit pixels are separately arranged as shown in FIG. 1. The 4T-2S step & repeat unit pixel 100 outputs a conversion voltage corresponding to the image signals generated in two photodiodes PD1 and PD2 using two transmission transistors M11 and M21 and three transistors MC2, MC3, and MC4 commonly shared.
The reset signal Rx12 applied to the gate of the reset transistor MC2 is enabled at a predetermined time point before either of the transmission control signal Tx1 or Tx2 applied to the gates of the transmission transistors M11 and M21 is enabled. The reference numeral “12” in the “Rx12” has been named for this reason. Similarly, the reference numeral “12” of the selection signal Sx12 applied to the gate of the selection transistor MC4 has been named for the same reason.
As described above, since a single common image signal conversion circuit is shared by two unit pixels, the image signals applied to two unit pixels are converted into electrical signals using two photodiodes and five transistors M11, M21, MC2, MC3, and MC4 in the 4T-2S step & repeat unit pixel 100. Therefore, the number of transistors required for each pixel becomes 2.5.
When four 4T unit pixels commonly share a single image signal conversion circuit, the number of transistors required for each pixel is reduced to 1.75. If other conditions are the same, the area that can be occupied by the photodiode increases in comparison to the above case in which only two 4T unit pixels commonly share a single image signal conversion circuit. This will improve the aperture ratio of the image sensor.
FIG. 2 is a circuit diagram of a conventional 4T-4S step & repeat unit pixel.
Referring to FIG. 2, a 4T-4S step & repeat unit pixel 200 includes four photodiodes PD0 to PD3 for generating image chares corresponding to incident image signals, four transmission transistors M21 to M24 for switching image charges generated from four photodiodes, and a common image signal conversion circuit having three transistors M25, M26, and M27 for converting the detected image charges into corresponding electrical signals. Transmission control signals Tx01, Tx11, Tx00, and Tx10 are applied to the gates of the transmission transistors M21, M22, M23, and M24, respectively.
Considering a general photodiode array in an image sensor, the 4T-4S step & repeat unit pixel 200 is obtained by combining into a single cell two photodiodes used to detect a green component from the image signals, one photodiode used to detect a blue component, and one photodiode used to detect a red component. Referring to FIG. 2, two photodiodes PD0 and PD3 used to detect a green component are diagonally arranged. The photodiode PD1 used to detect a blue component and the photodiode PD2 used to detect a red component are also diagonally arranged.
When the 4T-4S step & repeat unit pixel 200 is implemented in practice, the photodiodes are arranged in the same locations as those shown in the circuit diagram of FIG. 2. Therefore, the 4T-4S step & repeat unit pixel 200 has a square structure obtained by combining four most adjacent photodiodes.
FIG. 3 is a circuit diagram of another conventional 4T-4S step & repeat unit pixel.
Referring FIG. 3, the 4T-4S step & repeat unit pixel 300 includes four photodiodes PD0 to PD3 for receiving image signals, four transmission transistors M21, M22, M23, and M24 for switching image charges generated from the photodiodes PD0 to PD3, and a common image signal conversion circuit M25, M26, and M27 for converting the detected image charges into corresponding electrical signals. The transmission control signals Tx00, Tx10, Tx20, and Tx30 are applied to the gates of the transmission transistors M21, M22, M23, and M24, respectively.
When the 4T-4S step & repeat unit pixel 300 is implemented in practice, the photodiodes are arranged in the same locations as those shown in the circuit diagram of FIG. 3. Therefore, the 4T-4S step & repeat unit pixel 200 has a structure obtained by combining four photodiodes vertically disposed into a single cell. The 4T-4S step & repeat unit pixel 300 has a rectangular structure, and transmission control signals Tx00, Tx10, Tx20, and Tx30 are applied to the gates of the transmission transistors M21, M22, M23, and M24, respectively, connected to the four photodiodes.
Comparing the 4T-4S step & repeat unit pixels 200 and 300 of FIGS. 2 and 3 with each other, the 4T-4S step & repeat unit pixel 200 of FIG. 2 is formed by combining a total of four photodiodes arranged in two horizontal lines, each of which has two photodiodes, whereas the 4T-4S step & repeat unit pixel 300 of FIG. 3 is formed by combining four photodiodes arranged in series along a single vertical line. For this reason, the 4T-4S step & repeat unit pixel 200 of FIG. 2 requires the transmission control signals to be applied to two lines 0 and 1, while the 4T-4S step & repeat unit pixel 300 of FIG. 3 requires the transmission control signals to be applied to four lines 0, 1, 2, and 3.
As described above, since two photodiodes used to detect a green component are diagonally arranged, one of the photodiodes should be arranged in a first vertical line, and the other photodiode should be arranged in a second vertical line as shown in FIG. 2. Therefore, in the 4T-4S step & repeat unit pixel 200 of FIG. 2, proposed by Micron Technology, Inc., the photodiode for detecting a green component and the photodiode for detecting a red or blue component are alternately enabled. Accordingly, there may be errors in processing the green component, which is considered as a most important color component.
Generally, when a certain unit pixel is failed, a recovery algorithm may be applied to the image signals detected from other neighboring unit pixels in order to compensate for the failure as if it normally operates. On the other hand, since the 4T-4S step & repeat unit pixel 300 of FIG. 3, proposed by SAMSUNG Electronics, Inc., has a rectangular structure having pixels vertically connected in series, a conventional recovery algorithm cannot be applied without change when any unit pixel is failed. Therefore, a processing circuit should be redesigned in consideration of the aforementioned layout difference, and the size of the image sensor also increases.